vmcore: introduce mmap_vmcore()

vmcore: introduce mmap_vmcore()

This patch introduces mmap_vmcore().

If flag MEM_TYPE_CURRENT_KERNEL is set, remapped is the buffer on the 2nd
kernel.  If not set, remapped is some area in old memory.

Neither writable nor executable mapping is permitted even with mprotect().
 Non-writable mapping is also requirement of remap_pfn_range() when
mapping linear pages on non-consequtive physical pages; see
is_cow_mapping().

On x86-32 PAE kernels, mmap() supports at most 16TB memory only.  This
limitation comes from the fact that the third argument of
remap_pfn_range(), pfn, is of 32-bit length on x86-32: unsigned long.

Signed-off-by: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>